namespace WcfService1
{

    [ServiceContract]
    public interface IService1
    {

        [OperationContract]
        [FaultContract(typeof(InvalidUsernameFault))]
        void RegisterUser(string username, string password, string email); 
    }

}

using System.ServiceModel;
using System.Runtime.Serialization;

namespace WcfService1
{
    [ServiceContract]
    public class BugTracker
    {

        [DataContract]
        public class InvalidUsernameFault
        {
            private string CustomError;
            public InvalidUsernameFault(string error) { CustomError = error; }
            public InvalidUsernameFault() { }

            [DataMemberAttribute]
            public string Message
            {
                get { return this.CustomError; }
                set { this.CustomError = value; }
            }

            [DataMemberAttribute]
            public string Reason
            {
                get { return this.CustomError; }
                set { this.CustomError = value; }
            }

        }

    }

}

using System.Runtime.Serialization;
using System.ServiceModel;

namespace WcfService1
{

    public class Service1 : IService1
    {

        public void RegisterUser(string username, string password, string email)
        {
            throw new FaultException<WcfService1.BugTracker.InvalidUsernameFault>(new WcfService1.BugTracker.InvalidUsernameFault());
        }

    }

        private void button1_Click(object sender, EventArgs e)
        {
            try { new Service1().RegisterUser("user", "pass", "email"); }
            catch (FaultException<InvalidUsernameFault> usernameException)
            {
                MessageBox.Show(usernameException.Message);
            }
        }